1. Field of the Invention
This invention relates to methods of measuring bone mineral density (BMD) with x-ray fluoroscopic imaging equipment, and in particular to methods of measuring BMD in the limbs or extremities of a human or other animal with so-called mini C-arm x-ray fluoroscopic imaging systems, as well as to apparatus therefor.
2. Description of the Prior Art
In present-day medical practice, x-ray based systems are widely employed for bone densitometry (including measurement of BMD) , e.g. to diagnose, evaluate and/or monitor osteoporosis. Apparatus and procedures for x-ray bone densitometry are described, for example, in U.S. Pat. Nos. 4,947,414 and 5,040,199, the complete disclosures of which are incorporated herein by this reference. X-ray bone densitometry has heretofore typically used a scanning pencil beam or fan beam of x-rays and a point detector or a linear array of point detectors, although the aforementioned U.S. Pat. No. 5,040,199 also describes use of a cone-shaped beam impinging on an x-ray image intensifier to produce an optical image that is viewed by a television camera. In general, known x-ray bone densitometers are dedicated systems of large weight and bulk that must be fixedly installed at permanent, stationary locations, occupying substantial area, although a system has been proposed that can fold so as to pass through a typical hospital room door. Pending U.S. patent application No. 08/484,568, filed Jun. 7, 1995 and assigned to the same assignee as the present application, describes and claims beam flattening in a fan-beam x-ray bone densitometer, and pending U.S. patent application No. 08/484,484, also filed Jun. 7, 1995, and assigned to the same assignee as the present application, describes and claims scanning sequences (with a fan beam) that include dual energy scans for BMD and single energy scans for imaging; both of these pending applications are incorporated herein by this reference.
As distinguished from densitometers, x-ray fluoroscopic imaging systems provide images of bone and tissue similar to conventional film x-ray shadowgrams but produced by conversion of an incident x-ray pattern to a "live" enhanced (intensified) optical image that can be displayed on a video monitor directly, i.e., essentially contemporaneously with the irradiation of the patient's body or body portion being imaged; the term "fluoroscopic imaging" is used herein to designate such provision of directly video-displayed x-ray images. In some x-ray fluoroscopic imaging systems, the entire system is carried on an easily movable cart and the x-ray source and detector are mounted on a rotatable mini C-arm dimensioned for examining smaller body parts such as the extremities (wrists, ankles, etc.) of a human patient. An imaging device, including an image intensifier, suitable for use in such a system is described in U.S. Pat. No. 4,142,101, the complete disclosure of which is incorporated herein by this reference; one illustrative example of a currently commercially available mini C-arm x-ray fluoroscopic imaging system is that currently sold under the trade name "FluoroScan III" by FluoroScan Imaging Systems, Inc., of Northbrook, Ill., a subsidiary of the assignee of the present application. Mini C-arm x-ray fluoroscopic imaging systems are economical in space, conveniently movable (as within a hospital, clinic or physician's office) to a desired temporary location of use, and offer superior safety (owing to low levels of electric current utilization and reduced exposure of personnel to scatter radiation) as well as ease of positioning the source and detector relative to a patient's extremity for imaging.